Hitherto-known techniques include estimating a charge status (SOC: State of Charge) of a secondary cell, such as a nickel-metal hydride secondary cell and a lithium ion secondary cell and controlling charge and discharge of the secondary cell in accordance with the thus-estimated SOC.
Patent Document 1 to be referred to hereunder discloses a battery management system comprised of: a sensing section for measuring a charge-and-discharge current flowing into a battery and a temperature and a terminal voltage of the battery; a prediction section for presuming an SOC of the battery by integration of the charge-and-discharge current; a data rejection section for generating data pertinent to an error attributable to a measurement model in accordance with a rate of change over time in at least any one of factors: the battery temperature, the charge-and-discharge current, the SOC, and charge-and-discharge current; and a measurement section for making a correction to the estimated SOC of the battery by utilization of the measurement model and the data pertinent to the error. To be specific, the data rejection section sets a circuit equivalent to a modeled battery as a measurement model and generates a gain commensurate with variance that is the error attributable to the measurement model. A Kalman gain is generated by utilization of the gain commensurate with variance that is the error attributable to the measurement model, and a correction is made to the estimated SOC by utilization of the thus-generated Kalman gain.
Specifically, the prediction section predicts the SOC of the battery, which is a state variable (x), and a voltage Vdiff applied to diffusion impedance by utilization of an equation of state. The prediction section generates a predicted state variable and covariance that is an error in prediction of the state variable, supplying them to the measurement section. The measurement section predicts a value that can be measured by utilization of the SOC predicted by the prediction section and the voltage Vdiff; namely, a terminal voltage of the battery. The measurement section employs a differential form of equation in order to solve nonlinearity between the SOC and an OCV and use the Kalman filter. The measurement section generates a Kalman gain in order to make a correction to the predicted SOC and the voltage Vdiff. The Kalman gain is set to value that minimizes the covariance.
Patent Document 2 describes detecting an SOC from a relationship between a charge voltage and a charge current acquired during recharge of a battery when the SOC is high and close to an upper limit of a control region and detecting an SOC from a relationship between a discharge voltage and a discharge current when the SOC is low and close to a lower limit of the control region.
Patent Document 3 describes reconfiguring an internal characteristic of an electrochemical cell by use of a mathematical battery model in applications for hybrid vehicles and electric vehicles. Specifically, the descriptions refer to establishing a model comprised of: a mathematical expression, which takes into account an interface temperature, of a reaction rate of an electrochemical reaction occurring in an interface between electrodes and an electrolyte; an electrical expression of spatial accumulation of electrical charges on each of the electrodes; a balance of electrical charges in each of a solid phase and a liquid phase (the electrolyte); a balance of substance in all phases of a system; and a balance of energy of the system that calculates a temperature of a rechargeable electrochemical system, and also refers to calculating time-varying fluctuations in internal electrochemical variable. The descriptions additionally refer to an electrical potential of they system being recorded as an output signal.